1. Field of the Invention
The present invention generally relates to a method and apparatus for semiconductor laser driving. In particular, the present invention relates to a method and apparatus for semiconductor laser driving capable of effectively reducing a time period for detecting differential quantum efficiency of a laser diode.
2. Description of the Related Art
A laser diode is a semiconductor laser which is small in size and performs a high-speed modulation driven by a drive current. In recent years, the laser diode has been widely used as a light source of an image forming apparatus such as a laser printer and so forth.
The laser diode generally employs one of two laser driving methods, which are a non-bias method and a bias method. A laser diode employing the non-bias method is driven by a pulsed current corresponding to an input signal, without a bias current. A laser diode employing the bias method is driven by a bias current and a pulsed current added to the bias current. The bias current applied to the laser diode has an amount smaller than that of a threshold current Ith of the laser diode, and the pulsed current corresponds to the input signal.
When compared with the laser diode with the non-bias method, the laser diode with the bias method may reduce a delay time required until a carrier density which may reach a level to cause a laser oscillation is obtained. That is, a delay time from a period the pulsed current corresponding to the input signal is applied to the laser diode to a period the laser diode emits a laser beam may be reduced. Consequently, the laser diode with the bias method has been generally used.
However, a background laser diode driving integrated circuit (LD driving IC) employing the above-described bias method uses a bias current that is fixed to a predetermined value. When such LD driving IC is used in a tandem color image forming apparatus and a multiple beam image forming apparatus including a plurality of laser diodes, threshold current values of respective laser diodes used in the apparatuses may not be consistent due to variations from production of the laser diodes or due to an operating temperature thereof. With such inconsistency in the threshold current values, the background LD driving IC cannot stably generate consistent optical pulse widths, resulting in a production of an inferior image quality.
High-resolution image forming apparatuses such as high-resolution printers, image forming systems using a 650-nm red laser diode, a 400-nm ultraviolet laser diode and so forth are now under development. These laser diodes need to take a relatively long duration to obtain a carrier density which may, in turn, generate a carrier density which may reach a level to cause a laser oscillation, when compared with conventional laser diodes having wavelengths of 780 nm and 1.3 μm. Thereby, the background LD driving IC using the bias method obtains a pulse having variations in pulse widths. That is, the pulse widths becomes narrower than a desired pulse width. Furthermore, with the recent developments of high-speed printers, narrower and more stable pulse widths are demanded for a desired width of an optical pulse.
To eliminate the above-described problem, differential quantum efficiency of a laser diode is taken into account. For stably generating consistent optical pulse widths, differential quantum efficiencies of laser diodes need to be detected. However, the time period required to detect the differential quantum efficiencies depend on characteristics of the laser diodes. The detection time period is generally determined according to a laser diode that takes the longest amount of time to detect the differential quantum efficiency. That is, the other laser diodes that have characteristics requiring a shorter detection time period need to keep pace with the laser diode taking the longest amount of time to detect the differential quantum efficiency. The above-described status has adversely affected a reduction of the detection time period of the differential quantum efficiency.